Add indexed call-site evidence to graph queries

[Widthdom]

Summary

• Add additive callsite_* evidence to references, callers, callees, and impact JSON output using persisted indexed source and exact reference coordinates.
• Preserve deterministic aggregation, bounded excerpts, truncation/recovery metadata, redaction, projection, and cursor behavior while avoiding source materialization for count or hidden-body output.
• Keep inspect field discovery aligned with fields it can actually materialize, and validate explicit impact --snippet-lines intent before query execution.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet test CodeIndex.sln -c Release --no-build --no-restore -p:UseSharedCompilation=false
  • net8.0: 11,731 passed, 7 skipped, 0 failed
  • net9.0: 11,252 passed, 423 skipped, 0 failed
• Focused Issue Add call-site-centered evidence to graph body previews #5099 / bounded-response / projection registry regression tests: 80 passed on each target framework
• dotnet run --project tools/CodeIndex.Changelog -- check (33 fragments validated)
• Two Codex adversarial review rounds completed; all actionable findings were addressed and retested.

Documentation and changelog

• Updated USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md in English and Japanese.
• Added changelog.d/unreleased/5099.added.md.

Follow-up candidates

• None.

Fixes #5099